1. Field of the Invention
The present invention relates generally to band limiting apparatus and more particularly to a slew rate converter for a video signal recording apparatus used in an input stage of, for example, a video tape recorder (VTR).
2. Description of the Prior Art
In a video signal recording apparatus such as a video tape recorder (VTR) and the like, a band limiting apparatus is used in order to remove an undesired high frequency component in the signal processing process.
FIG. 1 schematically shows an example of a video tape recorder that utilizes a low-pass filter (LPF) as such band limiting apparatus.
Referring to FIG. 1, a signal V.sub.1 formed of a luminance signal, which is processed by a so-called Y/C separator circuit (not shown), is applied to an input terminal 1. This input signal V.sub.1 is supplied to a low-pass filter (LPF) 2, in which the high frequencies are filtered out to provide an output signal V.sub.2. The output signal V.sub.2 from the low-pass filter 2 is supplied to a clamping circuit 3, in which it is clamped to keep a DC level of its sync. chip portion constant. A pre-emphasizing circuit 4 is provided in order to avoid a so-called triangular noise. An FM-modulator circuit 5 is provided to FM-modulate the output signal from the pre-emphasizing circuit 4. An FM-modulated output signal from the FM-modulator circuit 5 is amplified by a recording amplifier 6 and is recorded on a magnetic tape 8 by a recording head 7. An adding circuit for adding down-converted color difference signals and the like is not shown in FIG. 1.
Let it be assumed that fc represents the cut-off frequency of the low-pass filter 2 shown in FIG. 1. Then, as shown in FIG. 2, the frequency characteristic of the output signal V.sub.2 is attenuated by the same attenuation factor whether the level at frequency f.sub.c, say, is V.sub.A or V.sub.B.
However, in the region (i.e., the hatched portion 9 in FIG. 2) wherein the signal level of the output signal V.sub.2 is high and the frequency thereof is high, there is the substantial disadvantage that the deterioration of image quality such as so-called picture drop-out, an inversion of video image or the like occurs in the picture reproduced from the magnetic tape 8 due to the modulation characteristic of the FM-modulator circuit 5, the recording characteristic of the magnetic tape 8 and the like. This phenomenon is similar to a so-called over-modulation in the AM-modulator circuit and is therefore generally referred to as an over-modulation.
To avoid this over-modulation, as shown in FIG. 2, it is enough to lower the cut-off frequency fc of the low-pass filter 2. This proposal, however, removes a relatively high frequency component whose signal level is under about an intermediate level, resulting in the high frequency component of the reproduced picture being lost
More specifically, in the case of, for example, the video tape recorder, if a video signal maintains a flat characteristic up to a luminance signal corresponding to 100 IRE and can suddenly attenuate the frequency characteristic of the luminance signal in a range of from 100 IRE to 120 IRE, then it can be expected that an excellent image quality is provided.